Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

ePTFE Gland Packing - An Effective Sealing Element

Expanded PTFE (ePTFE) is a material that is constantly evolving. From simple mono-directional (uni-directional) gasket tapes, the material has been further worked into ePTFE membranes, multi-directional tapes and sheets and ePTFE Ropes.

Expanded PTFE (ePTFE) Gland Packing is becoming increasingly popular because of its incredible properties as a sealing element and the ease with which it can be used. Here we look at some of the variants and benefits of ePTFE Gland Packing.

Variants of ePTFE Gland Packing

Typically, we see two variants – braided ePTFE gland packings and pure ePTFE gland packings.

Braided gland packings, as the name suggests, uses thin filaments of PTFE/ePTFE yarn to make a braided square profile. This product possesses the properties of PTFE while having the added strength lent by the braiding process. The trade-off is that the material is not as compressible as pure ePTFE packing and is hence used only in applications where the pressure and torque being applied on the system are high enough to induce compression in the braided material.

Pure ePTFE gland packings are also referred to as PTFE Rope, ePTFE Rope and ePTFE Round Rope. It is a round cross section of PTFE that has been mono-directionally stretched to give compressibility to the material. The process is nearly identical to making ePTFE gasket tapes, although care needs to be taken to ensure that the outer surface of the rope remains smooth and that the material is handled carefully during processing to ensure no ovality develops in the shape. The benefit of using this material is that it has a far higher seal-ability and can offer sealing even at very low pressures and torques.

Both variants described above can be infused with fillers of glass and graphite to offer added properties such a creep resistance and improved tensile strength.

Applications of ePTFE Gland Packing

Like ePTFE Gasket Tape, gland packing finds uses in applications involving one of the following:

  1. High temperature environments

    The capability to withstand between -250°C and +250°C immediately makes ePTFE Gland Packing an essential sealing element in applications involving high temperatures. It remains true that very few materials combine high seal-ability with temperature resistance. While rubbers may offer sealing, they melt beyond 150-180°C. Similarly, some silicones have shown high resistance to temperature, but cannot form as effective a seal as ePTFE unless pressure are high.

    Apart from high temperatures, ePTFE is also the material of choice in cryogenic applications, since there is no risk that the material will become brittle and suffer a loss in properties at low temperatures.

    Further enhancements – by adding fillers of glass, carbon and graphite – can be introduced into the material to improve properties such as thermal expansion and creep. These allow for the properties of the ePTFE to be fine tuned to ensure the application parameters are being fully addressed.

  2. Chemically corrosive environments

    As an inert material, PTFE resists numerous chemicals and remains one the few materials that designers turn to in the event that a system may churn our corrosive elements that are unpredictable.

    Furthermore, while ePTFE is structurally a porous material, the pore sizes are so small that water is not absorbed into the material. ePTFE is hydroscopic and hence offers no risk that the fluids within the system will stagnate within the material.

  3. High pressure environments

    We have spoken above about pressure. A key feature of ePTFE is that even with low torques, it can offer sealing at high pressures. Most standard sealing materials will perform well at lower pressures, but once the load within the system increases, they may experience fatigue and failure.

    Certain systems – such as those using brittle materials such as glass, may not have the option of being tightened beyond a point, as this would crack the materials. In such cases, ePTFE is ideal in taking the load.

  4. Food grade and sterile environments

    PTFE is FDA approved in its resin form and certified for food grade applications. As a result, ePTFE gland packings can also be employed in applications where the fluids would be used in or come in contact with food.

    In addition to food, many labs that manufacture pharmaceuticals and specialised chemicals need an inert material that will not contaminate the final product in the even that there is contact. In such cases, making the sealing elements using PTFE and/or ePTFE is the only viable option.

Expanded PTFE (ePTFE) Gland Packing is usually supplied in rolls and in diameters ranging from 3mm up to 20mm. Rolls have a practically infinite shelf life, which means they can be stocked and kept for both installation and maintenance requirements.

For more information, please do visit us at www.polyfluoroltd.com

PTFE (Teflon) Tubing - Cost and Quality Concerns of Weaker Substitutes

It is not uncommon in an industry catering to high-performance polymers to frequently come up against the issue of pricing. Polymers such as PTFE, PEEK, PVDF and PPS command prices that are sometimes up to 20X the cost of cheaper plastics such as polypropylene or polyethylene. This results in suppliers sometimes passing off a low-grade polymer as the high-performance variant, with the client being none-the-wiser. There is also the very real and commercial issue that in many cases, it is the purchase department who procure the material, and their KRAs are tied closely with the cost of the product, rather than the authenticity. In addition to this, supplying a test certificate that claims the polymer is genuine is simple enough and so the material is passed without much ado.

It is only when the material fails, or when the client’s engineering teams takes the initiative to have the material tested in a lab that the true nature of the polymer becomes apparent. We have had clients come to us with material procured from local vendors that has failed miserably. The client – now concerned with quality – seeks clarification on the root cause of the failure. When we inform them that the material is not what they think it is, we are usually met with shock.

A similar pattern tends to follow specifically around PTFE tubing. For the most part, PTFE (Teflon) tubes look and feel like any other polymer. The colour of the tube ranges from milky white to translucent to nearly transparent (more on this later). This is not much different that the shades seen in tubes of nylon, polypropylene or polyurethane, to name just a few. In all these cases, the price of PTFE (Teflon) tubing is likely to be anywhere from 5X to 10X more expensive. Hence, many times the client will inform us that we need to check our costing, since their current procurement rates are far below what we have quoted.

In this article, we focus on both the cost benchmarks and quality metrics of PTFE (Teflon) tubes. We hope that with this information, a client would be able to better distinguish genuine PTFE (Teflon) tubes, from the cheaper variants that the market sometimes tries to push.

PTFE (Teflon) Tubes – Pricing Benchmarks

When we set out to install our PTFE paste extruder for making PTFE (Teflon) tubes, our first concern was competing with China. We had already witnessed the market for PTFE stock shapes (rods and sheets) become flooded with cheap – albeit low quality – Chinese material and did not want our product to suffer the same fate. Since nearly all PTFE tube is imported into India (we remain one of the very few with the technology to manufacture PTFE tubes in India), we procured import data from the preceding 2 years to analyse the landed cost of the imported material.

The table below shows the average price per Kg for PTFE (Teflon) Tubes from China, the US and Europe. Although as an engineering company, we rarely ever quote a client by weight, it is useful to compare the same to understand the costing.

 

China

Europe/US

Ex-works Price per Kg (US$)

31.0

45.1

Add: Freight @ US$1 Per Kg

1

1

Add: Customs Duties @ 15%

4.8

6.9

Landed Cost per Kg (US$)

36.8

53.0

Unsurprisingly, Chinese tubes are cheaper by over 30% when compare with Europe/US. This clearly does not consider any of the quality variations between the two sources or indeed between Chinese tubes and our own tubes. But having the China price – and the hard data to support this – has allowed us to refute the claims of clients who say they are buying PTFE (Teflon) tubes are US$10-15 per Kg.

We also found that our own tubes could be made for as low as 10% below the China price. Obviously, this depends highly on the grade od raw materials used. Since some grades – such as Daikin, DuPont and 3M need to be imported, our pricing is also impacted accordingly.

Therefore, our standard response when a client claims to be getting a lower price is to first quote the China price – which everyone agrees is about as low as the price can get – and then request the client for a sample of tube so we can verify whether it is PTFE or some other polymer. Our experience here has been fruitful. Genuine OEMs are not keen to use material that is inferior. They will usually respond positively to such an exercise and appreciate our efforts to supply authentic PTFE material. Traders and price players would rather ignore our data – which is an immediate warning signal for us to stay as far away from them as possible!

PTFE (Teflon) Tube – Variants and Substitutes

We mentioned above that tubes made from nylon, polypropylene and polyurethane are sometimes passed off as PTFE (Teflon) tubes. Notably, there is a vast difference in the basic properties of these materials (which is the reason PTFE is opted for in the first place). Most notably is the temperature capability. Quite simply, PTFE can withstand a temperature of 260°C without suffering any deformation or loss in properties. Since most of the cheaper polymers would melt at anywhere above 150°C, popping a small length of tube into an oven would be the easiest way to check the authenticity.

It should also be noted that PTFE (Teflon) tubes are rarely fully transparent. We will elaborate on this below but as a client, if someone is offering you low-cost PTFE (Teflon) tubes that have a glass-like transparency, you are most likely being sold a variant of polycarbonate or acrylic tubes.

In addition to substitute polymers, there exist variants within PTFE (Teflon) tubes that should also be investigated. The range of PTFE (Teflon) tubes described above gives us some clue as to the composition and quality of the tubes.

  1. Milky white tubes – good quality PTFE (Teflon) tubes should never appear milky white and opaque. This is a property of low-quality tubes – usually from China – and is a result of adding Titanium di Oxide to the PTFE. This both reduces the cost of the material and allows any dark spots or blemishes within the material to be masked. Property-wise, the additive diminishes the tensile strength of the material, leading to a lower burst pressure.

    Many traders will sometimes insist that milky-white tubes are genuine. They are sadly mistaken and have most likely been convinced of the same by a manufacturer wishing to squeeze out a lightly higher margin.

  2. Translucent tubes – most PTFE tubes made with good quality resins will appear translucent. With higher quality resins, there results in a bluish tinge in the tube. Suffice to say that if you observe these properties in your PTFE tubes, the quality is probably good.

  3. Transparent tubes – as mentioned above, this is a rare variant within PTFE (Teflon) Tubes. Transparency can only be attained if the resin is a ‘modified grade’. Companies such as Daikin, DuPont (Chemours) and even GFL have variants of extrusion grade resins which have been modified. This modified resin allows for the final tube to exhibit transparency. However, even with an extremely high grade of resin (we are told that the modified grade of Asahi Glass offers the highest clarity), the transparency would never be glass-like.

We hope that the above information gives a clear picture of the pitfalls when ordering PTFE (Teflon) tubes. It should be emphasized that PTFE (Teflon) tubes offer a set of properties unlike any other polymer. In addition to this, the techniques involved in processing the tube are both expensive and proprietary. All these factors contribute to the end-result that PTFE (Teflon) tubing is a high-end product and that getting it at a price comparable with a regular polymer should be viewed with caution.

Solar Tracker Bearings - Considerations for Design and Manufacture

With renewable energy showing no signs of slowing down, it has become of paramount importance to understand and adapt to the growing needs of this industry. The increase in outlay, combined with the aggressive targets that these projects set make it essential to have quick, ready solutions that are both scalable and replicable across projects.

Solar power plants – as we covered in an earlier article – are still on a path towards maximising efficiency. While it is true that some plants can now boast costs per unit below that of traditional power plants, this does not hold true across the industry. Furthermore, it is well known that without subsidies, many of these plants would not have been able to sustain themselves up to the point where they were no longer losing money operationally.

As one might imagine, solar plants are highly dependent on the extent to which they can maximise the exposure of their panels to the sun. It is not alone enough to have an ample quantum of sunlight. Without the panels at the right angles, the ‘harvest’ from the sun’s energy will remain low and the plant may not achieve the efficiencies needed to justify its operational viability.

We covered Solar Tracker Bearings in an earlier article, where we looked mainly at the composition of the material used. Since then, we have had the chance to explore both the design elements and manufacturing challenges that arise in this product. In addition to this, experience is showing us that the global industry is focussed on a few key sizes and developing these are the key to having a ready solution.

Load and stress considerations

The fundamental purpose of the solar tracker bearing is to bear the load of the square pipe on which the panels are fixed. This pipe is rotated by the solar tracker mechanism itself and the bearing also facilitates this rotation. The pipes are usually found in 90mm, 100mm, 120mm and 150mm sizes and these appear to be a standard across the industry.

Loads experienced by the bearing are typically vertical loads – the weight of the pipe – and radial loads as the bearings rotate while in contact with the clamps around them. It is therefore required to have bearings that can absorb this vertical load while having a low enough coefficient of friction to allow for smooth rotation.

Vertical loads could be in the range of 10-15KN (1-1.5 Tonnes). While this is a relatively low load, it is still vital to check that the bearing can accommodate it, before committing to the client.

We used 3D simulations of the assembly on our design of the bearing to check the stress points and displacements on the bearing. This is a part that would need to work over many years outdoors, so we needed to be sure it would not buckle under the loads. Our stress checks confirmed that given the loads we had defined, the part would experience a deflection of within 60 microns, which is well within the acceptable limits.

Part design considerations

As we covered before – one of the main criteria for the bearing is that it needs to be light-weight. The bearing weight feeds back into the efficiency of the solar plant. Heavier bearings imply a higher load on the solar tracker when it rotates the pipe. This in turns means a higher power consumption and a net energy loss to the system.

Keeping the bearings light-weight mean that the parts benefit from being injection moulded, not machined. Although we do have projects where we have supplied machined bearings, these are usually for trials and fitment. Once the bearings are needed in bulk, injection moulding not only allows for higher volumes, but also allows for designs such as the one we have shown above, ensuring a 60-70% weight (and therefore cost) saving over a machined part.

The mould for a given bearing would have a life of nearly 200-300K sets. Considering a single project requires anywhere between 20K and 30K sets, it is possible to develop a mould and use it over many projects.

In our experience, if the volume requirement exceeds 5K sets, it is more cost effective to develop a mould, rather than opt for the parts to be machined.

Material design considerations

We covered material composition in our earlier article. However, it is important to reiterate that the materials to be used must have the following properties:

  1. Light weight

  2. Low moisture absorption

  3. UV resistant

  4. High strength

  5. Low coefficient of friction

  6. Wear resistant

  7. Injection mouldable

Special formulations exist that offer the above properties. Being in control of the formulation allows the recipes to be tweaked based on a client’s requirements.

There is still much to learn about the solar tracker field, but with each project, our understanding and familiarity improves.